Nuclear magnetic response and conformational energy characterization of repeat peptides of elastin: the cyclohexadecapeptide, cyclo-(L-Val1-L-Pro2-Gly3-Gly4)4

Elastomeric Polypeptide Biomaterials: Structure and Free Energy Transduction

The primary, secondary, tertiary and quaternary structures are presented and discussed for elastomeric polypeptides capable of undergoing inverse temperature transitions, that is, these polypeptides fold with the extrusion of water on raising the temperature through a transition. The elastomeric polypeptides, which are comprised of repeating peptide sequences, appear to be dominantly entropic elastomers. As these elastomers exhibit preferred secondary, tertiary and quarternary structure, they are not properly characterized as the random chain networks commonly ascribed to entropic elastomers. Instead, a mechanism of damping of internal chain dynamics on extension is described and referred to as the librational entropy mechanism of elasticity. Indeed, there are emerging a set of structural concepts for elastomeric polypeptides.

Of particular interest is that these elastomers are capable of exhibiting free energy transduction, e.g., thermomechanical and chemomechanical. A principle is stated for thermomechanical transduction and a postulate is given for chemomechanical transduction which is supported by prediction and experimental verification. The underlying mechanism is considered to be an aqueous(hydration) mediated apolar(hydrophobic)-polar interaction free energy which arises out of a competition between apolar and polar groups for limited waters of hydration. In general, there emerges a simple structural perspective of free energy transduction in elastomeric polypeptide biomaterials that involves thermal, mechanical or chemical means of altering the equilibrium between folded and unfolded states, where the folded states are dynamic helices called β-spirals with dominantly hydrophobic intramolecular interturn contacts and the unfolded states have the hydrophobic groups exposed to water.

Circular dichroism and UV-resonance Raman investigation of the temperature dependence of the conformations of linear and cyclic elastin

We used electronic circular dichroism (CD) and UV resonance Raman (UVRR) spectroscopy at 204 nm excitation to examine the temperature dependence of conformational changes in cyclic and linear elastin peptides. We utilize CD spectroscopy to study global conformation changes in elastin peptides, while UVRR is utilized to probe the local conformation and hydrogen bonding of Val and Pro peptide bonds. Our results indicate that at 20 degrees C cyclic elastin predominantly populates distorted beta-strand, beta-type II and beta-type III turn conformations. At 60 degrees C, the beta-type II turn population increases, while the distorted beta-strand population decreases. Linear elastin predominantly adopts distorted beta-strand and beta-type III turn conformations with some beta-type II turn population at 20 degrees C. Increasing temperature to 60 degrees C results in a small increase in the turn population.

Structure and dynamics of two elastin-like polypentapeptides studied by NMR spectroscopy

The structure and dynamics of two synthetic elastin-like polypentapeptides, poly(G(1)V(1)G(2)V(2)P) and poly(AV(1)GV(2)P), were studied in D(2)O and H(2)O at various temperatures by using (1)H, (2)H,(13)C, and (15)N NMR spectra, relaxations, and PGSE self-diffusivity measurement. Signal assignments were made using COSY, NOESY, HXCORR, HSQC, HMBC, and SSLR INEPT techniques. Temperature-induced conformation changes were studied using (3)J(NHCH) couplings, NOESY connectivity, chemical shifts, and signal intensities. Hydrodynamic radii were derived from self-diffusion coefficients measured by the pulsed-gradient spin-echo (PGSE) method. Selective hydration (hydrophilic or hydrophobic) was explored using NOESY and ROESY spectral methods and longitudinal and transverse (1)H relaxation of HOD and quadrupolar (2)H relaxation of D(2)O. Four different physical states were discerned in different temperature regions for both polymers: state I of a rather extended, statistically shaped and fully hydrated polymer below the critical temperature (approximately 299-300 K); state II, a relatively coiled and globular but disordered preaggregation state, developing in a rather narrow region, 300-303 K, in the case of poly(AV(1)GV(2)P) and in a broader region, overlapping with the next one, in poly(G(1)V(1)G(2)V(2)P); state III, a tightly coiled, more compact state in the region 303-313 K; and, finally, state IV, an aggregated (and eventually flocculating and sedimenting) state beyond 313 K. States II-IV coexist in varying proportions in the whole temperature range above 299 K. A structure characterized by a beta-turn stabilized by H-bonding between the Ala carbonyl and Val(2) NH groups of poly(AV(1)GV(2)P) was detected by NOESY just above the transition temperature. States II and III are progressively more stripped of their hydration sheath but retain some molecules of water confined and relatively immobilized in their coils.

Self-assembly of bioelastomeric structures from solutions: mean-field critical behavior and Flory-Huggins free energy of interactions

Elastic and quasi-elastic light scattering studies were performed on aqueous solutions of poly(Val-Pro-Gly-Gly), a representative synthetic bioelastomer that differs from the previously studied poly(Val-Pro-Gly-Val-Gly) by the deletion of the hydrophobic Val in position four. When the spinodal line was approached from the region of thermodynamic stability, the intensity of light scattered by fluctuations, and the related lifetime and correlation length, were observed to diverge with mean-field critical exponents for both systems. Fitting of the experimental data allowed determining the spinodal and binodal (coexistence) lines that characterize the phase diagrams of the two systems, and it also allowed a quantitative sorting out of the enthalpic and entropic contributions to the Flory-Huggins interaction parameters. The contribution of valine is derived by comparison of the two cases. This can be viewed as sorting out the effect of a modulation of the solute. The same approach may allow sorting out the entropic and enthalpic effect of modulations of the solvent by cosolutes (or by cosolvents). This could be of particular interest in the case of small osmolytes, affording important adaptive roles in nature, at the cost of very limited changes in genetic information. Finally, the suggestion is further supported that statistical fluctuations of anomalous amplitude, such as those occurring in proximity of the spinodal line, have a role in promoting the process of self-assembly of extended supramolecular structures. On the practical side, the present approach appears useful in the design of novel synthetic model systems for bioelastomers.

Self-assembly of a bioelastomeric structure: solution dynamics and the spinodal and coacervation lines

The stability, metastability, and instability regions of aqueous solutions of a representative synthetic bioelastomeric polymer, poly (Val-Pro-Gly-Val-Gly), were determined by a combined use of elastic and quasi-elastic light scattering experiments. The approach followed here offers the attractive advantage of singling out the relevant contributions to the total scattering even in the presence of traces of noninteracting larger sized impurities. Conclusions so reached were checked by means of independent experiments. The present results provide descriptions of the very early events in the physics of bioelastogenesis in terms of general polymer science and phase transitions, and in terms of an unexpected possible functional role of density fluctuations.

Flexibility of tripeptides in solution: free energy molecular mechanics

A full theory of the conformations of biopolymers requires a method for treating the effects of solvent on the induced structures. This is especially critical in aqueous solvent where hydrogen-bonding and dielectric shielding play major roles in determining the relative stability of conformers. Calculations of peptide conformations on a free energy surface are contrasted with the traditional sort of calculations which employs a simple potential energy function (in vacuo). The method employs a pairwise decomposable free energy surface determined by approximate analytical statistical mechanical theory. Applications are presented for tripeptides of alanine and glycine in water. This method, with precomputed free energy functions, takes the same amount of time and effort as traditional molecular mechanics in vacuo.